Fainga'anuku was named among six loose forwards in the All Blacks' squad for next month's Nations Championship tests against Scotland, Wales and England, though there is every chance the explosive 27-year-old could be deployed on the wing or in the midfield as the occasion demands.
Fainga'anuku missed the first tranche of Nations Championship matches in July and the subsequent four-test tour of South Africa after sustaining a fractured fibula and high ankle sprain playing for the Canterbury Crusaders in Super Rugby Pacific.
More recently, his provincial campaign for Tasman wrapped up early with a head-knock and concussion in the final rounds, and he may not be fully fit for the opening test against Scotland in Edinburgh on November 7.
Rennie, however, is confident Fainga'anuku can make a difference when his time comes.
“It’s less (playing time) than we would have liked,” Rennie told New Zealand media on Tuesday of Fainga'anuku's disrupted buildup.
“Still, it’s better to have him in the group for the next four or five weeks. He’s in reasonable nick now.”
Having once starred on the wing for the Crusaders, Fainga'anuku made a successful switch to the back row this season.
With All Blacks captain Ardie Savea sidelined, along with season-ending injuries to Luke Jacobson and Shannon Frizell, Fainga'anuku takes his place ahead of test loose forward Du’Plessis Kirifi and impressive young flanker Semisi Ta'eiloa.
"It gives you massive flexibility," Rennie said of Fainga'anuku's versatility.
"You can name a 5-3 bench but it’s generally a 6-2 (split) and he can definitely make (an) impact in all of those spots."
Rennie will also have a first look at number eight Christian Lio-Willie, who debuted last year and played three tests under former head coach Scott Robertson.
He said there were serious discussions about taking Lio-Willie to South Africa before they opted for other players ahead of him.
"He's really tough. He's got a big engine, a really high skill-set and suits the type of game we're trying to play. We’re really excited for him," he said.
Still coming back from a calf injury, Richie Mo'unga was included in the tour squad with incumbent flyhalf Damian McKenzie and the uncapped Josh Jacomb.
Mo'unga had recovered quicker than hoped from his injury but the elite playmaker is more likely to play in the second test against Wales, rather than come straight back in for Scotland, Rennie said.
